The invention relates to liquid toners, and to producing a reduction in image drag (a term later defined herein) in electrostatic printing, particularly high speed electrostatic printing as performed by the apparatus and method disclosed in copending U.S. patent application Ser. No. 07/458,940 filed Dec. 29, 1989, now U.S. Pat. No. 5,043,749, entitled Printing Press and Method, and assigned to the assignee of this application.
Such a press employs (a) digital electronic image creation, generation, and merging; (b) electrophotographic printing, e.g. electrostatic printing of images using liquid toner for image development; (c) the type and versatility of web handling associated with forms presses or the like; (d) web feeding under controlled tension, which in turn contributes to accurate length control, an important factor in continuous forms manufacturing or other repetitive printing wherein the various pages must be of uniform length for further handling; (e) ability to print on a substantial variety of materials, of different thickness or other characteristics; and (f) ability to maintain quality electronically printed product at substantial speeds, in a range of at least 100 to 300 feet/minute (or even greater), and during speed changes within that range. It is to be noted that these speeds are substantially greater than modern electrophotographic copying machines, usually sheet fed, the fastest of which operate at speeds no greater than approximately 100 feet/minute.
The unique printing engine of such press utilizes a drum having a surface photoreceptor, e.g. a photoconductive surface as the active surface on which developed electrostatic images are created, and an offsetting arrangement by which these images are transferred to the forms material, most commonly a paper web. The drum is rotatably driven at a peripheral velocity equal to web speed through the press. Special high intensity charging, exposing-discharging, developing and cleaning systems assure the drum surface has a uniform electrostatic charge, of substantially high potential, applied to its photoreceptor surface each revolution, is selectively discharged by exposure to a radiation pattern to form a latent image, that image is developed with a liquid toner, and after the image is transferred to print receiving material, the surface is then cleared of residual toner, and charged again as the next revolution begins. It should be understood that preferably the photoreceptor surface is continuous, as disclosed in said application, and the aforesaid process occurs continuously as the drum surface progresses past the different stations of the printing engine.
A digital imaging device, preferably in the form of a relatively high intensity LED array mounted to extend transversely of the rotating drum surface, operates to discharge the background or non-image areas of the passing drum surface to a potential substantially lower than the potential of the uniform charge, by exposing individual dot areas to focused radiation at a predetermined frequency and intensity, whereby the remaining or image areas(s) comprise a latent electrostatic image of the printed portions of the form. The size of these dots or pixels provides an acceptably high resolution image for forms printing. The latent image then is carried, as the drum rotates, past a developing station wherein it is subjected to the action of a special high speed liquid toner developer, thus forming a developed or visible image with liquid toner particles, which image is thence transferred and fixed to the paper web or other material. The developer is a special proprietary combination of small particle size toner, having a plurality of fibers integrally extending therefrom, dispersed in a carrier liquid. The liquid developer supply system constantly recirculates developer through a specially constructed shoe, which is closely fitted to the moving drum surface, for example at a spacing of about 500 microns (0.020 inch).
The developer is monitored and refreshed as needed to maintain a predetermined concentration of toner particles in volatile carrier liquid. The developer shoe functions as an electrode which is maintained at a potential in the order of +500 to 600 V DC. Thus the negatively charged toner particles are introduced into the shoe and dispersed among electrical fields between the image areas and the developer electrode, on the one hand, and between the background or non-image areas and the developer electrode on the other hand.
Typically, the electrical fields are the result of difference in potential between the image areas and the developer electrode which causes the toner particles to deposit on the images areas, and between the background areas and the developer electrode which causes toner particles to migrate to the developer shoe in those areas. Thus, the electrical fields in the image and non-image areas are reversed, resulting in a high quality distinction between image and background, and good coverage of solid image areas. The tendency of toner particles to build up on the developer shoe or electrode is overcome by the circulation of liquid toner through the shoe back to the toner refreshing system, as explained in said co-pending application.
As the drum surface passes from the developer shoe, a reverse rotating metering roll, spaced parallel to the drum surface by about 50 to 75 microns (0.002 to 0.003 inches), acts to shear away any loosely attracted toner in the image areas, and also to reduce the amount of volatile carrier liquid carried by the web with the toner deposited thereon, and to scavenge away any loose toner particles which might have migrated into the background areas. This metering roll has applied to it a bias potential in the order of +200 to 600 V DC, varied according to web velocity.
Details of a suitable charging system are disclosed in U.S. Pat. No. 5,017,964 issued May 21, 1991. Details of a developer supply system are disclosed in U.S. Pat. No. 5,019,868 issued May 28, 1991. An example of such toner is disclosed in U.S. Pat. No. 4,794,651, particularly Example 1 therein. Details of the reverse metering roll system are also disclosed in said U.S. Pat. No. 5,019,868. The web path then leads t o an image transfer station where idler rollers guide the web material into contact with a band-like area across the drum surface. Behind the web path at this location is a transfer corotron to which is applied a high DC voltage. The web is driven at a speed equal to the velocity of the drum surface to minimize smudging or disturbance of the developed image on the drum surface, and to assure that the printed image is of the proper length. Typical web speeds for such a press are in the range of 100 to 300 ft./min. Both toner particles and liquid carrier transfer to the web, including carrier liquid on the drum surface in the background areas.
In operation of such a press, a condition which has been named "drag" has been observed under certain circumstances. Drag is an image defect characterized by the appearance of toner particles deposited in areas that should contain no information, following a printed area, e.g. the particle deposits are found downstream along the print receiving material or media from printed areas, in the direction of travel of the print receiving media, in a diminishing trail. The magnitude of this defect has been found to depend upon the size of the gap between the photoconductor drum and the reverse roller, the speed at which printing is taking place (i.e. print material or web speed), and upon the toner used. The amount of drag observed has also been found to respond to the electrical process parameters that delimit toner development, developer electrode bias and reverse roll potential are shown to have some effect. The product of drag is made visible as a relatively small amount of toner, it is a relatively low density image figure, and therefore factors effecting transfer will effect drag as well. If a paper shows poor transfer characteristics in general, e.g., papers that are rough, drag will be less apparent.
The profound speed dependence of noticeable drag has kept this condition from even being observed in prior liquid toner imaging machines. Since the press as disclosed in said copending application Ser. No. 07/458,940, now U.S. Pat. No. 5,043,743 prints much faster than copiers or similar devices that use liquid toner imaging, drag is a major concern in that press. The amount of drag has been observed to increase as the printing speed increases, all other things remaining constant.
Prior art liquid developers have, in general, exhibited a trend toward smaller particle sizes. Many prior art patents speak in terms of submicron particle sizes, ranging up to no more than 10 microns, and in many cases up to no more than 2 microns. By way of example only, prior U.S. patents disclosing such examples are U.S. Pat. No. 3,926,825 (from about 0.01 to 10 microns), U.S. Pat. No. 3,954,640 (5 microns or less), U.S. Pat. No. 4,307,168 (preferred range of from about 0.1 to about 2.0 microns), U.S. Pat. No. 4,357,406 (not larger than 10 microns), and U.S. Pat. No. 4,476,210 (typically 0.4 microns, maybe as broad as 0.1 to 1 micron). The trend has been to achieve very small particle sizes, mostly in a effort to improve image resolution.